Hexaferrites have coercive field strengths of the degree required for high density magnetic storage (45 to 120 KA.sup.-1). In their chemical resistance they are superior to metal pigments and in their thermostability they are superior to iron oxide pigments doped with CrO.sub.2 and Co. Hexaferrite pigments having a magnetoplumbite (M-) structure have already been proposed for use in high density magnetic recording carriers. In their pure form as finely divided pigments, they have coercive field strengths much too high (160 to 480 KAm.sup.-1) for recording purposes. They are usually doped with Co and Ti ions for obtaining suitable coercive field strengths. Such doping reduces both the saturation magnetisation (M.sub.s) and the thermal resistance as well as rendering the product less economical due to the use of cobalt which is expensive. Numerous hexaferrites with different structures are known (e.g. W, X, Y, Z, U) which have not found any application in magnetic recording media on account of their insufficient saturation magnetisation (B. F. Haberey, Fachdokumentation des Fortschritts 1971, special issue, Elektro-Anzeiger, publishers W. Girardet, Essen). See also Ferromagnetic Materials, Vol. 3, (1982), p. 395 ff. Ed. E. P. Wohlfarth, North-Holland Publishing Co., Amsterdam. W-hexaferrite would be of interest on account of its high saturation magnetisation but so far only the powders prepared by sintering followed by size reduction have been proposed, (JA-Appl. 84-106, 107; C.A. 1984, 101; 221237; JA 84-174, 530 C.A. 1984, 102-54951; JA 84-174, 531, C.A. 1984, 102-38462) and these do not fulfill the criteria of a magnetic pigment. If pigments are to be suitable for use as magnetic recording materials, they must fulfil the condition of being finely divided, with diameters from 0.5 .mu.m to 0.05 .mu.m, and of having a narrow particle size distribution.
Uniformly fine particles in the necessary pigment size range below 0.5 .mu.m cannot be obtained by the subsequent size reduction of ceramic frits. No pigment-like W-hexaferrites have hitherto been known. Hexagonal M-ferrites (magnetoplumbite) are prepared from metal borate melts (U.S. Pat. No. 3,716,630 and U.S. Pat. No. 4,493,874). By this method it has been possible to produce materials suitable for magnetic recording purposes but these materials still have insufficient saturation magnetisation and a disadvantageous change in coercivity with increasing temperature. Another disadvantage of M-ferrites is the necessity of using cobalt for adjusting the coercivity to within a suitable range.
The known processes for the preparation of finely divided hexaferrites for signal storage are directed to the formation of anisometric, in particular platelet shaped particles, from which advantageous magnetic properties are expected if the platelet form is pronounced.
In the platelet shaped hexagonal ferrites described, the magnetically light direction is perpendicular to the plane of the platelets, with the result that for vertical recording the maximum storage density is limited by the diameter of the platelets. Furthermore, with conventional methods of preparation of magnetic storage media, only moderate orientation in the preferential magnetic direction can be achieved due to the rheological characteristics of dispersions of platelet shaped pigments.
It is an object of the present invention to provide hexagonal ferrites suitable for magnetic recording which are superior in their properties to the hexagonal ferrites known in the art and can be obtaind economically.